1. Field of the Invention
The present invention relates to a reflective liquid crystal display (LCD) apparatus having a reflection plate for reflecting to outside a light which has passed through a liquid crystal layer from outside.
2. Description of the Related Art
As compared to a transmitting LCD apparatus, a reflective LCD apparatus can reach a reduced power consumption and a thin type with a reduced weight and accordingly, has been used mainly in a portable terminal. In the reflective LCD apparatus, light incident from outside is reflected by a reflection plate in the apparatus so as to be utilized as a display light source, thereby eliminating need of back light.
The conventional reflective LCD apparatus has a basic configuration composed of a liquid crystal of the TN (twisted nematic) type, a single deflection plate type, STN (super twisted nematic) type, GH (guest-host) type, PDLC (high molecule dispersion) type, or cholesteric type, a switching element for driving the liquid crystal, and a reflection plate arranged inside or outside liquid crystal cells. The reflective LCD apparatus utilizes an active matrix drive method capable of realizing a high-resolution and high-quality image by using a thin film transistor (TFT) or metal/insulation film/metal structured diode (MIM) as a switching element, to which a reflection plate is attached.
FIG. 36 is a cross sectional view of a conventional reflective LCD apparatus of a single deflection plate type. Hereinafter, explanation will be given with reference to this figure.
An opposing substrate 1 includes a deflection plate, a phase difference plate 32, a glass substrate 4, a color filter 5, and a transparent electrode 6. A lower substrate 7 includes a glass substrate 8, a thin film transistor (TFT) 9 of an inverse stagger structure as a switching element formed on the glass substrate 8, an insulator protrusion 10 as a base for forming a, a polyimide film 11 as an inter-layer insulation film formed thereon, and a reflection electrode 13 connected to a source electrode 12 of the TFT and functioning as a reflection plate and a pixel electrode. A liquid crystal layer 14 is arranged between the opposing substrate 1 and the lower substrate 7.
A reflected light 16 is utilized as a light source. An incident light 15 from outside passes through the deflection plate 2, the phase difference plate 3, the glass substrate 4, the color filter 5, the transparent electrode 6, and the liquid crystal layer 14, and is reflected by the reflection electrode 13 to become the reflected light 16.
This reflective LCD display apparatus should have a display performance for displaying a bright and white display when the liquid crystal is in the light transmitting state. In order to realize this display performance, it is necessary to effectively eject forward the incident light 15 from various directions. For this, the polyimide film 11 is formed with the convex/concave structure, so that the reflection electrode arranged thereon can have a scattering function. Accordingly, control of the convex/concave structure of the reflection electrode 13 is the important factor in deciding the display performance of the/reflective LCD apparatus.
FIG. 37 and FIG. 38 show a production method of a conventional reflective LCD apparatus in cross sectional views. Hereinafter, explanation will be given with reference to these figures.
In the thin film transistor production procedure, firstly, a gate electrode 21 is formed on the glass substrate 20 (FIG. 37[a]). Next, a gate insulation film 22, a semiconductor layer 23, a doping layer 24 are formed (FIG. 37[b]). Next, an island 25 of the semiconductor layer 23 and the doping layer 24 is formed (FIG. 37[c]), and the source electrode 26 and the drawing electrode 27 are formed (FIG. 37[d]). After this, the reflection electrode is formed.
For forming the reflection electrode, firstly, an organic insulation film 28 having photosensitivity is formed (FIG. 37[e]). Then, photolithography is performed to form a protrusion 29 in the reflection electrode forming region (FIG. 37[f]), which is then melted by heating so as to be formed into a smooth protrusion 30 (FIG. 38[g]). Next, the protrusion is covered by an organic insulation film 31 to obtain a further smooth convex/concave surface 32 (FIG. 38[h]). Next, a contact portion 33 is formed for electrically connecting a reflection electrode to a source electrode of the thin film transistor (FIG. 38[i]), and then the reflection electrode 34 is formed (FIG. 38[j]). The method for forming this reflection electrode is disclosed, for example, in Japanese Patent Publication (examined) 61-6390 or in Tohru Koizumi and Tatsuo Uchida, Proceeding of the SID, Vol. 29, 157, 1988.
As has been described above, in the conventional reflective LCD apparatus, the convex/concave structure is formed by organic insulation film or inorganic insulation film having photosensitivity as a base which is covered by an organic insulation film or inorganic insulation film.
However, below the protrusions, there are formed a metal wiring, an electrode, a switching element, and the like, which are exposed to an etching liquid used in the etching procedure for forming the protrusions. As a result, a reaction between the etching liquid and the undercoat film deteriorates characteristic of the switching element and the remaining etching liquid lowers reliability of the switching element.
Moreover, when using an organic insulation film or inorganic insulation film having no photosensitivity for the insulation film below the reflection electrode, a photo resist pattern is formed on the insulation film and dry etching is performed to form a convex pattern. In this case, the undercoat film is exposed to plasma during the etching and the plasma damage deteriorates the characteristic of the switching element.
On the other hand, the conventional method for producing the conventional reflective LCD apparatus requires a number of production steps as has been described above. This increases the production cost, which in turn increases the cost of a reflective LCD apparatus. The reason why the reflective LCD apparatus requires a number of production steps is that a high-performance switching element and a high-performance reflection plate are formed on the same insulation substrate in order to obtain a bright high-quality display, and that the production of the high-performance reflection plate requires a method capable of forming the convex/concave structure on the reflection plate surface with a desired configuration. Accordingly, the conventional reflective LCD apparatus requires a number of film formation steps, photoresist (PR) steps, and etching steps.
Currently, no effective means is employed to simplify the production procedure. The convex/concave structure below the reflection electrode is currently produced as follows. Firstly, a photosensitive resin is applied, which is then patterned by an exposure step and a development step so as to form a convex pattern. However, in the area other than the portion having this convex pattern, the photosensitive resin film is completely removed. After this, the convex pattern is subjected to a thermal treatment so as to obtain a smooth protrusion shape, which is then covered by an organic insulation layer so as to obtain a desired smooth convex/concave surface.
That is, the insulation film below the reflection electrode consists of two layers: a film of convex shape and a film covering it. This insulation film has a function as an inter-layer insulation film for electrically insulating the reflection electrode from the switching element and the wiring. After this, a contact hole is formed in this insulation layer. Then, a metal thin film such as aluminum is layered thereon. This metal thin film is patterned to obtain a reflection electrode along the fine convex/concave structure of the insulation film.
Thus, formation of the reflection electrode has required five steps: (1) formation of an insulation film for forming a protrusion as a base; (2) formation of a protrusion; (3) formation of a contact hole; (4) formation of a metal thin film having a high reflection efficiency; and (5) formation of a reflection electrode.